coords is an array of the 3D coordinates for the nodes on
the local processor. So if there are 108 equation on a processor
for a displacement finite element discretization of elasticity (so
that there are 36 = 108/3 nodes) then the array must have 108
double precision values (ie, 3 * 36). These x y z coordinates
should be ordered for nodes 0 to N-1 like so: [ 0.x, 0.y, 0.z, 1.x,
... , N-1.z ].